1. End of Cosmic Dark Ages: Observational Probes of Reionization History Xiaohui Fan University of Arizona New Views Conference, Dec 12, 2005 Collaborators: Strauss, Becker, White, Richards, Schneider, Gunn, Carilli et al.

2. From Avi Loeb reionization

3. Reionzation simulation: Courtesy of N. Gnedin Late, phase transition By stellar sources

4. Three stages Pre-overlap Overlap Post-overlap From Haiman & Loeb

5. Reionization is an Interplay of Structure formation –WMAP cosmology Star formation and feedback –First stars –Enrichment of IGM Radiative transfer We expect it to be a complex process…

6. Open Questions: When: Early or Late –z~6: late –z~15: early How did reionization proceed: –Phase transition or gradual? –Once or twice? –Homogeneous or large scatter? What did it: –AGN? –Star formation? –Decay particles? Observational goals –Map the evolution of ionization state: neutral fraction (f HI ) vs. redshift –Find highest redshift galaxies and quasars: source of reionization

7. The Highest Redshift Quasars and Galaxies SDSS i-dropout Survey: –By late 2005: 6600 deg 2 at z AB <20 –Nineteen luminous quasars at z>5.7 –z max =6.42 Dropout and Ly  emission galaxies –z spec < 6.6 –z phot ~ 7 - 8 GRBs –050904 z=6.30

8. Keck/ESI 30min exposure  Gunn-Peterson (1965) effect in z=6.28 Quasar: increasing HI absorption IGM turning neutral? end of reionization? Keck/ESI 10 hour exposure  White et al. 2003 Becker et al. 2001

10. Evolution of Lyman Absorptions at z=5-6  z = 0.15

11. Accelerated Evolution at z>5.7 Optical depth evolution accelerated –z<5.7:  ~ (1+z) 4.5 –z>5.7:  ~ (1+z) >11 Dispersion of optical depth also increased –Some line of sight have dark troughs as early as z~5.7 –But detectable flux in ~50% case at z>6 –End of reionization is not uniform, but with large scatter (1+z) 4.5 (1+z) 11 Fan et al. 2005

12. Evolution of Ionization State UV Ionizing background: –UV background declines by close to an order of magnitude from z~5 to 6.2 –Increased dispersion suggests a highly non-uniform UV background at z>5.8 From GP optical depth measurement, volume averaged neutral fraction increase by ~ order of magnitude from z~5.5 to 6.2 Fan et al. 2005 Neutral fraction UV background

13. Gunn-Peterson Troughs Appearance of GP troughs show large line of sight variations –J1148 (z=6.42): detectable flux in , ,  : IGM highly ionized –J1030 (z=6.28) and J1623 (z=6.22) : no flux in , ,  Upper limit on neutral fraction –If IGM largely neutral, GP damping wing will wipe out all HII region transmissions –Existence of transmission at z>6 places an upper limit of average neutral fraction –f HI < 30%

14. Evolution of HII region Size Around Quasars Size of quasar HII region R s ~ (N Q t Q / f HI ) 1/3 Size of quasar HII region decreases by a factor of ~2.4 between z=5.8 and 6.4 Neutral fraction increased by a factor of ~14 over this narrow redshift range Fan et al. 2005 z em Haiman, Mesinger, Wyithe, Loeb et al. HII region size (Mpc) redshift

15. No Evolution in Ly  Galaxy LF at z>6 Neutral IGM has extended GP damping wing --> attenuates galaxy Ly  emission line Lack of LF evolution --> IGM neutral fraction < 30% - 50% (consistent with GP measurements) But systematics due to galaxy clustering and radiative transfer Malhotra & Rhoads; Hu et al. z=5.7z=6.5

16. Probing the first metals? GP effect saturates at z>6 Metal absorption line detected at z>6 Using metal lines to probe reionization? Pettini et al., Songaila White et al. 2003

17. Metal Absorption Line Probe: OI 1302 Hydrogen absorption (Gunn-Peterson) saturates at z>6 –Use metal lines? Oh (2002) –Use OI 1302 line –With almost identical ionization potential as HI -> same neutral fraction? –Low abundance: probing high neutral fraction –Predict OI forest during reionization Becker et al. (2005) –Detection of strong OI systems at z>6 –No OI forest –Consistent with high ionization at z~6 –Uncertainties in IGM enrichment history. Becker et al. 2005

18. GRBs as Probes of Reionization Detected to z=6.30 Advantages: –Bright –Flat K-correction due to time dilation at high-z –Small surrounding HII regions: could use damping wing of Gunn- Peterson trough to probe high neutral fraction Constraining neutral fraction –How to distinguish internal absorption from IGM damping wing?? –Using 050904: f HI < 0.6 (2- sigma) by fitting both DLA and IGM profiles Damping wing? GRB050904 Kawai et al. 2005

19. Summary of IGM Measurements IGM evolution accelerated at z>6 –Neutral fraction increased by order of mag from z=5.5 to z>6 –f HI a few percent –IGM temperature evolution IGM evolution is not uniform –~order of mag fluctuation in large scale UV background IGM is not mostly neutral at z~6 –Transmission spikes in GP trough –Lack of evolution in Ly  galaxy LF z~6 marks the end of overlapping stage of an inhomogeneous reionization

20. WMAP: early reionization Thompson scattering of CMB photons by free electrons during/after reionization --> polarization and large scale TE correlation WMAP first year: –  = 0.17 +/- 0.04 –Larger signal comparing to late reionization model Kogut et al. 2003

21. WMAP: early reionization Inconsistent with a phase transition at z=6 Early reionization at z=15 - 20 However, no direct conflict to Gunn-Peterson result, which is sensitive only to ~1% neutral IGM Overlapping could still be at z~6 IGM has complex reionization history  Feedback from Galaxy Formation Gnedin 2004 z reion = 6

22. Reionization History with Feedback all models normalized to  e = 0.17 H 2 -cooling redshift electron fraction Photo-heatingMetal Enrichment By introducing highly efficient star formation at z>10, and feedback effect to subsequent star formation, models can fit both GP and polarization results Haiman and Holder 2003

23. What ionized the Universe: AGNs, Star Formation or Else Exponential decline of quasar density at high redshift, different from normal galaxies Richards et al. 2005, Fan e al. 2005 SFR of galaxies Density of quasars Bouwens et al.

24. Reionization by AGNs? Can quasars do it? –No too few quasars Can low-luminosity AGNs ionize the IGM by z~6? –Stacking X-ray image of LBGs in UDF… too few faint AGNs Can accretion to seed BHs ionize the IGM by z~15? –Dijkstra, Haiman & Loeb (2004) –Soft X-ray background overproduced if quasars produce ~10 photons/H atom –‘Preionization’ to f(HI)~50% by X- rays is still allowed Fan et al. 2003

25. Reionization by stellar sources? Bouwens et al 2005 Bunker et al 2004 Necessary for reionization 6<z<9 (Stiavelli et al 2003) Large uncertainties in reionization photon budget: –IGM clumpiness –UV radiation and escape efficiency –Large cosmic variance in deep field data –Galaxy luminosity function at high-z

26. Summary When? –Reionization started early (z~15 - 20, WMAP) –And ended late (z~6, Gunn-Peterson) How? –History likely complex –Not a sharp phase transition in time feedback from galaxy formation –Not homogeneous in space large scale structure What? –Likely UV photons from star formation –Hard photons from accretion could still pre-ionize to low-level at high-z

27. What’s Next? GRB, JWST Planck, 21cm WMAP